Speed up warmServer by loading pages (and files) asynchronously (#16752)

* Async `new Page`

* Update pages.js

* Update pages.js

* Update pages.js

* Update pages.js

* Update pages.js

* Using mapLimit

* Update pages.js

* Test updates

* Update pages.js

* Move exists to page class

* Test fixes

* Slightly faster localized paths process

tests/content/crowdin-config.js

@@ -4,6 +4,8 @@ const ignoredPagePaths = config.files[0].ignore
 const ignoredDataPaths = config.files[2].ignore
 
 describe('crowdin.yml config file', () => {
+  jest.setTimeout(60 * 1000)
+
   let pages
   beforeAll(async (done) => {
     pages = await loadPages()